This invention relates to polymer compositions having improved bonding performance. In particular, the subject invention pertains to a polymer composition comprising a blend of a polypropylene polymer and a high molecular-weight (i.e. low melt index or melt flow) ethylene polymer. The subject invention further pertains to the use of the polymer blend composition which has improved bonding performance in various end-use applications, especially fibers, nonwoven fabrics and other articles fabricated from fibers (e.g., disposable incontinence garments and diapers). The fibers have good spinnability and provide a fabric having good bond strength and good elongation.
Fiber is typically classified according to its diameter. Monofilament fiber is generally defined as having an individual fiber diameter greater than 15 denier, usually greater than 30 denier per filament. Fine denier fiber generally refers to a fiber having a diameter less than 15 denier per filament. Microdenier fiber is generally defined as fiber having less than 100 microns diameter. The fiber can also be classified by the process by which it is made, such as monofilament, continuous wound fine filament, staple or short cut fiber, spun bond, and melt blown fiber.
A variety of fibers and fabrics have been made from thermoplastics, such as polypropylene, highly branched low density polyethylene (LDPE) made typically in a high pressure polymerization process, linear heterogeneously branched polyethylene (e.g., linear low density polyethylene made using Ziegler catalysis), blends of polypropylene and linear heterogeneously branched polyethylene, blends of linear heterogeneously branched polyethylene, and ethylene/vinyl alcohol copolymers.
Of the various polymers known to be extrudable into fiber, highly branched LDPE has not been successfully melt spun into fine denier fiber. Linear heterogeneously branched polyethylene has been made into monofilament, as described in U.S. Pat. No. 4,076,698 (Anderson et al.), the disclosure of which is incorporated herein by reference. Linear heterogeneously branched polyethylene has also been successfully made into fine denier fiber, as disclosed in U.S. Pat. No. 4,644,045 (Fowells), U.S. Pat. No. 4,830,907 (Sawyer et al.), U.S. Pat. No. 4,909,975 (Sawyer et al.) and in U.S. Pat. No. 4,578,414 (Sawyer et al.), the disclosures of which are incorporated herein by reference. Blends of such heterogeneously branched polyethylene have also been successfully made into fine denier fiber and fabrics, as disclosed in U.S. Pat. No. 4,842,922 (Krupp et al.), U.S. Pat. No. 4,990,204 (Krupp et al.) and U.S. Pat. No. 5,112,686 (Krupp et al.), the disclosures of which are all incorporated herein by reference. U.S. Pat. No. 5,068,141 (Kubo et al.) also discloses making nonwoven fabrics from continuous heat bonded filaments of certain heterogeneously branched LLDPE having specified heats of fusion. While the use of blends of heterogeneously branched polymers produces improved fabric, the polymers are more difficult to spin without fiber breaks and/or dripping at the spinneret die.
U.S. Pat. Nos. 5,294,492 and 5,593,768 (Gessner), both incorporated herein by reference, describe a multiconstituent fiber having improved thermal bonding characteristics composed of a blend of at least two different thermoplastic polymers which form a continuous polymer phase and at least one noncontinuous polymer phase. In the claims, Gessner recites that the at least one noncontinuous phase occupies a substantial portion of the surface of the fiber made from the blend. But while we believe the claims in U.S. Pat. Nos. 5,294,492 and 5,593,768 specify, for example, a core-sheath configuration with respect to the polymer phases, the photomicrograph (FIG. 1 therein) shows an island-sea type phase configuration for the fiber cross-section. Further, we believe it is the continuous polymer phase (not the noncontinuous phase) which occupies a substantial portion of the surface of the fiber exemplified (but not claimed) by Gessner. Also, all of the Examples (and presumably FIG. 1 therein) consist of polypropylene polymer blended with ASPUN(trademark) fiber grade LLDPE resins having a 12 or 26 g/10 minute 12 melt index as supplied by The Dow Chemical Company. The exemplar polypropylene polymer used by Gessner was described a xe2x80x9ccontrolled rheologyxe2x80x9d PP (i.e. a visbroken PP) having a melt flow rate of 26 and at least 90 percent by weight isotacticity.
U.S. Pat. No. 5,549,867 (Gessner et al.), incorporated herein by reference, describes the addition of a low molecular weight (i.e. high melt index or melt flow) polyolefin to a polyolefin with a molecular weight (Mz) of from 400,000 to 580,000 to improve spinning. The Examples set forth in Gessner et al. are all directed to blends of 10 to 30 weight percent of a lower molecular weight metallocene polypropylene with from 70 to 90 weight percent of a higher molecular weight polypropylene produced using a Ziegler-Natta catalyst.
U.S. Pat. No. 4,839,228 (Jezic et al.), incorporated herein by reference, describes biconstituent fibers having improved tenacity and hand composed of a highly crystalline polypropylene polymer with LDPE, HDPE or preferably LLDPE. The polyethylene resins are described to have a moderately high molecular weight wherein their I2 melt index is in the range of from about 12 to about 120 g/10 minutes.
Also, fibers made from blends of visbroken polypropylene polymer and homopolymer high density polyethylene (HDPE) having an I2 melt index of equal to greater than 5 g/10 minutes are known. Such blends are thought to function on the basis of the immiscibility of the olefin polymers.
WO 95/32091 (Stahl et al.) discloses a reduction in bonding temperatures by utilizing blends of fibers produced from polypropylene resins having different melting points and produced by different fiber manufacturing processes, e.g., meltblown and spunbond fibers. Stahl et al. claims a fiber comprising a blend of an isotactic propylene copolymer with a higher melting thermoplastic polymer.
WO 96/23838, U.S. Pat. No. 5,539,056 and U.S. Pat. No. 5,516,848, the disclosures of which are incorporated herein by reference, teach blends of an amorphous poly-xcex1-olefin of Mw greater than 150,000 (produced via single site catalysis) and a crystalline poly-xcex1-olefin with Mw less than 300,000, (produced via single site catalysis) in which the molecular weight of the amorphous polypropylene is greater than the molecular weight of the crystalline polypropylene. Preferred blends are described to comprise about 10 to about 90 weight percent of amorphous polypropylene. The described blends are said to exhibit unusual elastomeric properties, namely an improved balance of mechanical strength and rubber recovery properties.
U.S. Pat. No. 5,483,002 and EP 643100, the disclosures of both of which are incorporated herein by reference, teach blends of a semi-crystalline propylene homopolymer having a melting point of 125 to 165xc2x0 C. and a semi-crystalline propylene homopolymer having a melting point below 130xc2x0 C. or a non-crystallizing propylene homopolymer having a glass transition temperature which is less than or equal to xe2x88x9210xc2x0 C. These blends are said to have improved mechanical properties, notably impact strength.
Crystalline polypropylenes produced by single site catalysis have been reported to be particularly suited for fiber production. Due to narrow molecular weight distributions and low amorphous contents, higher spinning rates and higher tenacities have been reported. But, isotactic PP fibers, in general (and particularly when produced using single site catalyst) exhibit poor bonding performance.
U.S. Pat. No. 5,677,383 (Lai et al.), incorporated herein by reference, discloses blends of (A) at least one homogeneously branched ethylene polymer having a high slope of strain hardening coefficient and (B) at least one ethylene polymer having a high polymer density and some amount of a linear high density polymer fraction. The Examples set forth by Lai et al. are directed to substantially linear ethylene interpolymers blended with heterogeneously branched ethylene polymers. Lai et al. describe the use of their blends in a variety of end use applications, including fibers. The disclosed compositions preferably comprise a substantially linear ethylene polymer having a density of at least 0.89 grams/centimeters3. But Lai et al. disclosed fabrication temperatures only above 165xc2x0 C. In contrast, to preserve fiber integrity, fabrics are frequently bonded at temperatures less than 165xc2x0 C. such that all of the crystalline material is not melted before or during the fiber bonding step.
While various olefin polymer compositions have found success in a number of fiber and fabric applications, the fibers made from such compositions would benefit from an improvement in bond strength, which would lead to stronger fabrics, and accordingly to increased value to the nonwoven fabric and article manufacturers, as well as to the ultimate consumer. But any benefit in bond strength must not be at the cost of a detrimental reduction in spinnability and fiber elongation nor a detrimental increase in the sticking of the fibers or fabric to equipment during processing.
We have discovered that the inclusion of a high molecular weight ethylene polymer into a polypropylene polymer provides a multiconstituent fiber and calendered fabric having an improved bond performance, while simultaneously maintaining excellent fiber spinning and elongation performance. Accordingly, the subject invention provides a fiber having a diameter in a range of from 0.1 to 50 denier and comprising:
(A) from about 0.5 percent to about 25 weight percent (by weight of the fiber) of at least one ethylene polymer having:
i. an I2 melt index less than or equal to 10 grams/10 minutes, preferably less than 5 g/10 minutes, more preferably less than or equal to 3 g/10 minutes, most preferably less than or equal to 1.5 g/10 minutes, especially less than or equal to 0.75 g/10 minutes and
ii. a density of from about 0.85 to about 0.97 grams/centimeters3, as measured in accordance with ASTM D792, (or a corresponding percent crystallinity in range of about 12 to about 81 percent by weight, as determined using differential scanning calorimetry (DSC)), and
(B) a polypropylene polymer, preferably a polypropylene polymer having a melt flow rate (MFR) in the range of about 1 to about 1000 grams/10 minutes, measured in accordance with ASTM D1238 at 230xc2x0 C./2.16 kg, more preferably in range of about 5 to about 100 grams/10 minutes,
with the proviso that where the ethylene polymer is an ethylene/xcex1-olefin interpolymer having an I2 melt index in the range of about 5 to about 10 g/10 minutes, the density of the ethylene/xcex1-olefin polymer is greater than 0.87 g/cm3, preferably greater than or equal to 0.90 g/cm3, and more preferably greater than or equal to 0.94 g/cm3, as measured in accordance with ASTM D792,
with the proviso that where the ethylene polymer is an ethylene homopolymer or ethylene/xcex1-olefin interpolymer having a density greater than or equal to 0.94 g/cm3, as measured in accordance with ASTM D792, the I2 melt index of the ethylene polymer is less than 5 g/10 minutes, preferably less than or equal to 3 g/10 minutes, more preferably less than or equal to 1.5 g/10 minutes, most preferably less than or equal to 0.75 g/10 minutes, and wherein the fiber is thermal bondable at 340 pounds/linear inch and a bond roll surface temperature in the range of 127 to 137xc2x0 C.
In a particular aspect, the subject invention provides a fiber having a diameter in a range of from 0.1 to 50 denier, a continuous polymer phase and at least one discontinuous polymer phase which comprises:
(A) as the at least one discontinuous polymer phase, from about 0.1 percent to about 30 weight percent (by weight of the fiber) of at least one ethylene polymer having:
i. an I2 melt index less than or equal to 10 grams/10 minutes, and
ii. a density of from about 0.85 to about 0.97 grams/centimeters3, and
(B) as the continuous polymer phase, a polypropylene polymer,
with the proviso that where the ethylene polymer is an ethylene/xcex1-olefin interpolymer having an I2 melt index in the range of about 5 to about 10 g/10 minutes, the density of the ethylene/xcex1-olefin polymer is greater than 0.87 g/cm3 (or has a DSC percent crystallinity greater than 13 weight percent), preferably greater than or equal to 0.90 g/cm3 (or has a DSC percent crystallinity greater than 33 weight percent) and more preferably greater than or equal to 0.94 g/cm3 (or has a DSC percent crystallinity greater than 60 weight percent),
with the proviso that where the ethylene polymer is an ethylene homopolymer or ethylene/xcex1-olefin interpolymer having a density greater than or equal to 0.94 g/cm3, the I2 melt index of the ethylene polymer is less than 5 g/10 minutes,
wherein, prior to any bonding operation, the continuous polymer phase constitutes more than 50 percent of the fiber surface area and the two polymer phases cross-sectionally provide an island-sea configuration, and wherein the fiber is thermal bondable at 340 pounds/linear inch and a bond roll surface temperature in the range of 127 to 137xc2x0 C.
In specific embodiments, the discontinuous phase constitutes an amount of the fiber surface area which is within or less than 50 percent, preferably 25 percent, more preferably 10 percent of amount contained in the blend composition. That is, in such embodiments, the surface area percentage of the discontinuous phase polymer is insubstantial as it closely approximates the total composition weight percentage of the discontinuous phase polymer), as determined using an electron microscopy technique which may include selective staining to enhance resolution.
Preferably, the fiber of the invention will be prepared from a polymer blend composition comprising:
(A) at least one homogeneously branched ethylene polymer, more preferably at least one substantially linear ethylene/xcex1-olefin interpolymer having:
i. a melt flow ratio, I10/I2,xe2x89xa75.63,
ii. a molecular weight distribution, Mw/Mn, defined by the equation:
MwMn,xe2x89xa6(I10/I2)xe2x88x924.63, and 
iii. a critical shear rate at onset of surface melt fracture of at least 50 percent greater than the critical shear rate at the onset of surface melt fracture of a linear ethylene polymer having about the same I2 and Mw/Mn, and
which constitutes the discontinuous polymer phase, and
(B) at least one isotactic polypropylene propylene.
The subject invention further provides a method for improving the bonding strength of a fine denier fiber comprised of at least one polypropylene polymer, the method comprising providing in an intimate admixture therewith less than or equal to 22 weight percent, preferably less than or equal to 17 weight percent, more preferably less than or equal to 12 weight percent of at least one ethylene polymer having a density of from about 0.85 to about 0.97 g/cm3 and an I2 melt index of from about 0.01 to about 10 grams/10 minutes, with the proviso that where the ethylene polymer is an ethylene/xcex1-olefin interpolymer having an I2 melt index in the range of about 5 to about 10 g/10 minutes, the density of the ethylene/xcex1-olefin polymer is greater than 0.87 g/cm3 and with the proviso that where the ethylene polymer is an ethylene homopolymer or ethylene/xcex1-olefin interpolymer having a density greater than or equal to 0.94 g/cm3, the I2 melt index of the ethylene polymer is less than 5 g/10 minutes.
The subject invention further provides a polymer composition having improved bond strength comprising:
(A) from about 0.1 percent to about 30 weight percent (by weight of the composition) of at least one ethylene polymer having:
i. an I2 melt index less than or equal to 10 grams/10 minutes, and
ii. a density of from about 0.85 to 0.97 grams/centimeters3, and
(B) a polypropylene polymer,
with the proviso that where the ethylene polymer is an ethylene/xcex1-olefin interpolymer having an I2 melt index in the range of about 5 to about 10 g/10 minutes, the density of the ethylene/xcex1-olefin polymer is greater than 0.87 g/cm3 and with the proviso that where the ethylene polymer is an ethylene homopolymer or ethylene/xcex1-olefin interpolymer having a density greater than or equal to 0.94 g/cm3, the I2 melt index of the ethylene polymer is less than 5 g/10 minutes.
The subject invention further provides a polymer composition of the invention, in the form of a fiber, fabric, nonwoven or woven article, rotomolded article, film layer, injection molded article, thermoformed article, blow molded article, injection blow molded article, or extrusion coating composition.
The inventive fibers and fabrics can be produced on conventional synthetic fiber or fabric processes (e.g., carded staple, spun bond, melt blown, and flash spun) and they can be used to produce fabrics having high elongation and tensile strength, without a significant sacrifice in fiber spinnability. As an unexpected surprise, the polymer blend exhibits excellent fiber spinnability even though the ethylene polymer is characterized as having a high molecular weight. In fact, excellent polymer blend spinnability is achieved even where the ethylene polymer itself is not spinnable into fine denier fibers (that is, diameters less than about 50 denier) when used alone.
It is also surprising that improved bond strength is obtained without commensurate reductions in elongation performance.
It is a further surprise that relative to known PP/HDPE blends, improved bond strengths are obtained at relatively low polymer densities and crystallinities.
It is still another surprise that inventive blends based on high molecular weight ethylene/aromatic vinyl interpolymers provide dramatically improved bond strengths relative to comparative blends based on ethylene/xcex1-olefin interpolymers having comparable crystallinities and melt indexes.
As another surprise, the invention where the polypropylene polymer (B) is manufactured using a metallocene or single-site or constrained geometry catalyst system results in substantially stable bond strengths at about 340 pli in the bonding temperature range of from about 127 to about 137xc2x0 C.
These and other embodiments are more fully described in the detailed description in conjunction with the following figures.